In many instances, it is desirable to coat metallic surfaces so as to protect the surfaces from corrosive environments. For instance, automobile underbodies are coated to protect them from road salt compounds. Marine vessel parts are coated to protect them from the marine air. In those instances, a polymer coating is applied to the entire surface of the substrate, or, in the alternative, a prepolymer coating is applied to the substrate and then polymerized in toto.
In addition, it is also desirable to selectively protect certain areas of a metallic surface. One such instance is the selective protection of metallic surfaces being processed as circuit traces for electronic circuit boards. In those instances, only certain preselected areas of the metallic surface are coated by a polymeric film.
Specifically, circuit traces can be made by using either of two photoresist systems in a coating, imaging, developing and etching process. In the first system, the substrates or metallic surfaces are coated with a negative working photoresist which polymerizes upon exposure to actinic radiation. In the second system, the substrate or metallic surface is coated with a positive acting photoresist which becomes soluble in developer solution upon exposure to actinic radiation.
Once either one of the resists have been applied to the surface, the surface is then exposed, to actinic radiation. When using the negative resist, the surface is exposed to radiation through a photographic negative bearing an image of the desired circuit. As a result, the sections of the resist exposed by the radiation become photopolymerized and thus less soluble in a developer solution. Then during the development phase, those sections of resist which were shielded from the radiation, and which thus remain substantially unpolymerized, are dissolved away by means of a suitable solvent that does not dissolve the photopolymerized sections. This stage is known as development because it develops an image of the circuit by uncovering certain sections of the metallic surface. After development, the uncovered metal surface is etched to form a printed circuit. The photopolymerized resist may then be stripped chemically by means of a solvent, leaving a circuit pattern formed from the unetched metal surface.
When using the positive resist, the coated surface is exposed to radiation through a positive image of the desired circuit. The exposed areas of the coating are thus rendered more soluble and subsequently removed in a developing solvent. As done when using the negative resist, the metal surfaces left uncovered during development are etched, thus leaving a positive image of the desired circuit.
Photoresists have become important tools when preparing circuit boards having plated through holes. Such holes are being increasingly used as circuit boards are increasingly being made with two conductive sides. The additional conductive surface increases the boards' capabilities.
The above-described two sided boards are conventionally made from a laminate consisting of copper/epoxy/copper sheets. Each copper side of the laminate has a circuit etched onto it. The two sides are connected electrically, as required for the particular circuits involved, by small apertures or "through holes". Other terms used in the art are "component holes" or "vias." Through holes, as initially drilled or otherwise formed, are not electrically conductive because of the intervening insulating epoxy layer. Accordingly, the holes' interiors must be coated with copper to electrically connect the two copper sheets. This copper coating can be applied by electroless copper deposition, thus forming one type of plated through hole (PTH). Another type of PTH includes those holes which have copper electrolytically deposited thereon after the initial electroless deposition of copper. See Norman S. Einarson, Printed Circuit Technology (published by Printed Circuit Technology, 1977); Fisher, G. L.; Sonnenberg, W., & Bernards, R.; "Electroplating of High Aspect Ratio Holes " Printed Circuit Fabrication, Vol. 12, No. 4 (April, 1989) pp.39-66; D'Ambrisi, J. J. et al., "The Chemistry of Plating Small Diameter Holes", Part II; Printed Circuit Fabrication, Vol. 12, No. 8 (Aug., 1989) pp.30-42.
Some of the difficulties in making circuit boards begin with those copper-coated holes. After the holes are created and plated with copper, the laminate is subjected to the etchant operations that tend to attack the copper coatings within the holes. Accordingly, the art has developed various methods for protecting the copper plated holes.
Two methods of protection include (1) paraffin plugs for the holes and (2) tenting with dry and liquid photoresist films. However, paraffin plugs are difficult to handle because of problems in removing the plugs when they are no longer needed. In addition, while tenting has been used with greater success, the problems attendant upon tenting also make its use somewhat awkward.
Tenting works by protecting the plated holes with a dry film comprising a photopolymerizable sheet. The areas of the sheet which cover and protect the holes are exposed to UV radiation and polymerized. The circuit board is then later processed with the plated holes remaining covered by the "tent". The tents are then later removed by proper solvents.
As recognized by those skilled in the art, proper photoimaging of the circuit requires that any protective photopolymerizable coverings used in the process be extremely thin. As a result, when using tenting techniques, the polymerizable tents are thin. However, because of their thinness, these tents tend to be weak and thus are subject to tearing, breaks or "pinholes." Once those flaws form, solvents or etchants can seep through and come in contact with the copper deposited on the through holes' sides, thus destroying the interconnect between the two copper sheets. The same problem occurs when the dry film is laid down improperly, thus allowing etchant solutions and developers to leak underneath the film.
In addition to film flaws, tenting results in the formation of "annular rings" or shoulders on the board's surface. These rings result from the requirement that a tent's diameter must be larger than the hole diameter in order to provide an attachment point for the tent and to afford hole protection. Their formation occurs during the etching process, because, after etching, the tent has not only protected the inner lining of the aperture, but has also protected an annular portion of the metal surface surrounding the aperture.
However, annular rings have become a problem as the trend towards smaller circuit boards and higher density circuits continues. For example, as circuit boards get smaller there will be less space available for the rings on the boards' surfaces. Thus, there will be less surface area onto which the tents can be anchored. Moreover, as more and more circuit traces are designed into the boards, large annular rings are more and more becoming a hindrance to design around. For example, there are some instances where multiple fine line circuits are run parallel and one of the circuit traces will require a connection to the other side of the board. In such an instance, a hole will be required. However, if the annular ring for that hole is too large, the other traces must be diverted away from the hole to avoid a short-circuit. Thus, to avoid designing around the ring, it would be ideal to eliminate their presence altogether.
Further, it is generally recognized that the current resin/photoactive functionality combinations found in common photoresists are capable of resolving smaller features. On the other hand, the current liquid and dry film photoresists do not maximize those capabilities in that to form finer features with a good manufacturing yield it is generally recognized that thinner films with fewer defects than those provided by current liquid and dry film photoresist application methods are needed. However, other known application methods, such as lamination, roll coating, flood screen printing, spraying, dip coating, curtain coating, etc., fail as an appropriate application method as the film thickness decreases below1 mil. For instance, in order to resolve features as small or smaller than 4 mils, which is currently the state of the art, it is preferable that the film thickness of the protective covering used be 25% or less of the feature size being resolved. However, when attempting to obtain thicknesses less than 1 mil with those methods, a significant number of defects begin to appear in the coating that results.
One method that avoids the above problems is the electrodeposition of a photoresist. For instance, a photoresist coating is applied to the substrate by applying an electric charge to the substrate, which in turn attracts a charged photoresist. See U.S. Pat. Nos. 4,632,900 to Demmer et al. and 3,954,587 to Kokawa. Thus, in electrodeposition a thin film of resist forms directly onto the surfaces of the plated through holes and thus avoids the awkward tenting process of laying the film down, the attendant annular ring formation and the film flaw problem. However, electrodeposition involves additional equipment and time to construct the electrodeposition apparatus. Electrodeposition also consumes electrical power and requires charged resins. Even further, photoresist compositions usually have optimal component ratios at which the components should be applied to the surface. However, by using electrodeposition, preferential deposition of certain charged particles may alter the ratio of components actually deposited. Thus, a method which has the advantages of electrodeposition baths in that the substrate can be effectively coated and protected, but which also avoids the problems encountered when using electrodeposition methods would be desirable.